Magnetic sensors, such as monopoles, are used extensively to sense the rotating speed of a turbine shaft or other turbomachinery components. Monopoles operate on the principle that the magnetic field generated by a permanent magnet located in the monopole is attracted by any ferromagnetic object moving near the pole piece. Shifting magnetic lines cut across the monopole coil inducing an output voltage proportional to the speed of the moving object (rate of change of flux lines). These monopoles make use of what is known as "stray magnetic field" and no provision for a "return magnetic circuit" is required.
Shaft revolutions per minute (RPM) can easily be determined by sensing the moving teeth of a gear mounted on the shaft according to the following formula: ##EQU1## where F is the frequency of the monopole generated signal in Hertz, and N is the number of teeth on the gear.
The conventional methods of testing magnetic sensors such as monopoles is by checking their output voltage under conditions similar to those encountered during actual operation. This requires set-ups using various gears, expensive high speed motor drives, and time-consuming air-gap and speed adjustments.
Since each type of monopole requires a different set-up, testing with gears is quite expensive; because of the individual mounting of each monopole and the adjustments required, the testing is also time-consuming. The difficulty in adjusting the air-gap between the gears and the monopole and in controlling motor speed, together with the problem of voltage modulation due to eccentricity of the gear and motor shaft, result in a substantial increase in the errors that can be introduced into the output voltage readings. In addition, the output voltage waveform is dependent upon the shape of the gear teeth and the length of the air gap. A safety hazard is also presented by the high speed rotating parts.